Exposure (photography)
In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor.
For example, a long exposure refers to a single, long shutter cycle to gather enough dim light, whereas a multiple exposure involves a series of shutter cycles, effectively layering a series of photographs in one image.
In a very simple model, for example, out-of-range values would be recorded as "black" (underexposed) or "white" (overexposed) rather than the precisely graduated shades of colour and tone required to describe "detail".
Methods for dealing with this situation include: using what is called fill lighting to increase the illumination in shadow areas; using a graduated neutral-density filter, flag, scrim, or gobo to reduce the illumination falling upon areas deemed too bright; or varying the exposure between multiple, otherwise identical, photographs (exposure bracketing) and then combining them afterwards in an HDRI process.
In manual mode, the photographer adjusts the lens aperture and/or shutter speed to achieve the desired exposure.
Exposure time is controlled in a camera by shutter speed, and the illuminance depends on the lens aperture and the scene luminance.
An approximately correct exposure will be obtained on a sunny day using ISO 100 film, an aperture of f/16 and a shutter speed of 1/100 of a second.
For example, the photographer may prefer to make his sunny-16 shot at an aperture of f/5.6 (to obtain a shallow depth of field).
Once the photographer has determined the exposure, aperture stops can be traded for halvings or doublings of speed, within limits.
The true characteristic of most photographic emulsions is not actually linear (see sensitometry), but it is close enough over the exposure range of about 1 second to 1/1000 of a second.
The film manufacturer's data sheets should be consulted to arrive at the correction required, as different emulsions have different characteristics.
Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying shutter or diaphragm) made in quick succession.
Slide/Transparency film has a narrow latitude in both highlight and shadow areas, requiring greater exposure accuracy.
In digital images this information loss is often irreversible, though small problems can be made less noticeable using photo manipulation software.
Areas of a photo where information is lost due to extreme darkness are described as "crushed blacks".
Digital capture tends to be more tolerant of underexposure, allowing better recovery of shadow detail, than same-ISO negative print film.
